Weird behavior from '66 Bassman 50 head. Need help.

[DexVegas]

AB165, 220k 'terminal strip' resistors removed. Just recapped it (filter, bias and bypass caps only) and began testing it. Cap job restored the volume I had noticed reduced (had not played it in many years).

Problem 1.: When volume and treble controls are maxed, sound is cut off. Certain manipulations of those control can get it motor-boating like crazy. Only occurs on normal channel.

Problem 2.: While playing, only one power tube was doing the 'purple pulse' thing along to my picking (I don't recall if both did before, but that is the case with my Marshall head).

The tone controls seem to have little effect overall, the bright switch does not seem to do anything.

Have tried tubes in all positions and used an old pair and combinations of both. The bias balance adjustment effects nothing even at it's extremes (no values known-can't find my meters gater-clip probes).

Any ideas will be much appreciated! TIA.

Great board by the way. Has been a huge help in the past.

[martin manning]

Do you know which circuit your amp was built to?

Have you cleaned/lubed the pots? Loss of signal could be due to dirt on the elements. Dioxit fader lube is the preferred product for this.

Get idle voltages at all the tube pins and post them. That will often point out a problem area.

[ToneMerc]

AB165, 220k 'terminal strip' resistors removed. Just recapped it (filter, bias and bypass caps only) and began testing it. Cap job restored the volume I had noticed reduced (had not played it in many years).

Problem 1.: When volume and treble controls are maxed, sound is cut off. Certain manipulations of those control can get it motor-boating like crazy. Only occurs on normal channel.

Problem 2.: While playing, only one power tube was doing the 'purple pulse' thing along to my picking (I don't recall if both did before, but that is the case with my Marshall head).

The tone controls seem to have little effect overall, the bright switch does not seem to do anything.

Have tried tubes in all positions and used an old pair and combinations of both. The bias balance adjustment effects nothing even at it's extremes (no values known-can't find my meters gater-clip probes).

Any ideas will be much appreciated! TIA.

Great board by the way. Has been a huge help in the past.

If this is an AB165 and if all you did was remove the 220K feedback resistors, it will not work correctly.

If you are going to get rid if the 220K feedback(positive FB network) resistors, you need to wire the power tube grid/bias section and bias supply like an AA864 and then transpose the primary leads of the output transformer.

TM

[martin manning]

If this is an AB165 and if all you did was remove the 220K feedback resistors, it will not work correctly.

If you are going to get rid if the 220K feedback(positive FB network) resistors, you need to wire the power tube grid/bias section and bias supply like an AA864 and then transpose the primary leads of the output transformer.

TM I don't think there is any positive feedback going on in the AB165. The 220k's are local negative feedback, and if the 47k + 0.1u were positive FB I think it would oscillate. That part of the circuit should probably be converted to feed back to the tail of the PI like the AA864: Delete the 0.1u and change the 47k FB resistor to 820R, then connect that to the top of a 100R inserted between the PI tail and ground. Swapping the either the OT primaries or secondaries will be required, but I don't see any need to change the bias circuit unless there is a desire to get rid of the balance control. Personally I'd keep it and add another trimmer to set the level.

A Bassman 50 is different from an AB165, so I'm confused...

[Cameron]

If this is an AB165 and if all you did was remove the 220K feedback resistors, it will not work correctly.

If you are going to get rid if the 220K feedback(positive FB network) resistors, you need to wire the power tube grid/bias section and bias supply like an AA864 and then transpose the primary leads of the output transformer.

TM I don't think there is any positive feedback going on in the AB165. The 220k's are local negative feedback, and if the 47k + 0.1u were positive FB I think it would oscillate. That part of the circuit should probably be converted to feed back to the tail of the PI like the AA864: Delete the 0.1u and change the 47k FB resistor to 820R, then connect that to the top of a 100R inserted between the PI tail and ground. Swapping the either the OT primaries or secondaries will be required, but I don't see any need to change the bias circuit unless there is a desire to get rid of the balance control. Personally I'd keep it and add another trimmer to set the level.

A Bassman 50 is different from an AB165, so I'm confused...

Reverse the wires to the speaker jack...flip the black and green. You will see its wired backwards for this circuit.

[DexVegas]

Not a Bassman50, sorry, should have said Bassman 50watt head. Sticker code either a QD or OD so '66 was wrong, circuit listed as AB165. Dude I got it from said it was '66 (that was in 1984, cost $50. He was a navy electronics tech and had put in a 3 prong cord, replaced all but one filter cap, also replaced one bypass cap, bias cap, and two signal caps). Used it as my main amp till 1989, been retired since.

Pulled out of retirement last week, getting to old to hump a half stack everywhere. Replaced filter caps with new F&T's, bypass/bias caps all atoms. Performed two mods I found in a Premier Guitar mag article ('Bringing A Blackface Bassman In From The Cold', December 2011). Removed the 500pf cap in parallel with the 100k plate resistor that is connected to the plate of V3. Also removed the 220k terminal strip resistors. Tried the 2nd mod from the article but restored it (sounded awful).

Have cleaned the pots but have not yet lubed them. Only cuts out if both volume and treble pots are taken over about the 9 mark. Turning either back below 9 restores the signal. Will post pin voltages asap.

Reverse the wires to the speaker jack...flip the black and green. You will see its wired backwards for this circuit.

What effect will this have? Need to know what to expect. No other mods have been done, 'cept those listed above, so swapping the primary leads of the output transformer is not needed.

Thank you!

[martin manning]

Swapping the OT output leads has the same effect as swapping the primary leads, i.e. it inverts the OT output signal with respect to the preamp output/phase inverter input signal. You would only need to do this if you move the global negative feedback to the PI tail, ala AA864, like I described above.

I don't know what to make of your being able to restore the signal with either the treble or the volume pot. How did you clean them?

[Cameron]

Swapping the OT output leads has the same effect as swapping the primary leads, i.e. it inverts the OT output signal with respect to the preamp output/phase inverter input signal. You would only need to do this if you move the global negative feedback to the PI tail, ala AA864, like I described above.

I don't know what to make of your being able to restore the signal with either the treble or the volume pot. How did you clean them?

Right......but in this circuit.....the transformer output wires are switched not the primary.

[Structo]

Do you guys that use later model Bassman amps all use a 12AT7 phase inverter?